Indeed, .bss starts at $2200.
The first 256 bytes of $2000 is the zero page. You can declare stuffs there using the .zp directive.
Next comes the stack ($2100 - $21ff included). There's no directive. You can still read/write there but you'll most likely shoot yourself in the foot as it's used to store returns addresses, and everything you pushed there with the pha/phx/phy/etc... instructions.